Blender Git Loki

Cycles: Experiment with culling objects based on the camera frustum

The idea is to give artists a simplier way to control memory usage in such
scenes as grass fields by doing automatic object culling based on whether
they're visible in the frame or not.

This is controlled on per-object level. In order to use this option few
steps are required:

- Allow renderer to use camera culling (Performance panel of render settings)
- Set camera cull margin (measured in relative value to the render resolution)
- Enable Camera Cull for objects which are desired to be culled
(object culling option could be found in Option panel in object buttons).

Interface decisions are not final for sure and to be refined if this
option is considered good enough for production.

Reviewers: juicyfruit, brecht, campbellbarton

Subscribers: venomgfx, eyecandy

Differential Revision: https://developer.blender.org/D1230